Preparation of anhydrous caustic



May 9, 1933- G. LYNN ET AL l 1,907,988

PREPARATION OF ANHYDROUS CAUSTIC Filed June 4, 1931 PRfJSU/Pr /A/ /A/cwfs VAcv/a/w Enea/Werff?) ATTO R N EYS Patented May 9, 1933 UNITED Ns'rA'rI-:s PATENT ,y OFFICE GEORGE LYNN, 0F WADSWORTH, AND

RALPH WAnsWoRTH MILLER, or BARBERTQN,

OHIO, .ASSIGNORS T0 PITTSBURGH PLATE GLASS COMPANY, 0F PITTSBURGH, PENN- SYLVANIA, A` CORPORATION 0F `IPENNSYLVANIA .PREPARATION OF ANI-IY!)ROU'S` CAUSTIC Application filed .Tune 4,

This invention relates to the production of solid anhydrous caustic soda from solutions thereof, its object being to provide a method of production Which is more economical and less dangerous than the usual practice and which at the same time yields a product of higher purity With novel and advantageous physical characteristics.

Further objects of the invention are in part obvious and in part Will appear more in detail hereinafter.

In the drawing, the View is a diagram illus trating conditions essential to proper practice of the present invention, now to be described in detail.

Our invention is based upon vacuum evaporation of caustic soda solutions, thereby gaining alladvantages inherent from vacuum evaporation, not only avoiding high temper aturcs and consequent increased cost, but also decreasing corrosion of apparatus and danger to Workmen. 7e have discovered that vacuum evaporation of caustic soda solutions, if properly performed, can be so conducted as to necessarily or inevitably cause the forma tion of anhydrous caustic soda crystals. It is adependable process to that en Our investigations have revealed the conditions necessary for dependable crystal formation, the results of such investigations in practice being illustrated in the drawing, Where temperatures in degrees Fahrenheit have been plotted as abscissas and partial pressures of Water Vapor, or standard vacuum, in inches of mercury, as ordinates. We have found that the temperature pressure relationship of boiling NaOH solution is represented by curve a; that of 60% NaOH solution by curve b; of 70% by curve cgand so on. More important, We have discovered, after scores of trials in practice, that these constant composition` lines terminate in a line A at Which crystalline anhydrous caustic soda is in equilibrium With the solution. That is to say, if atizero vacuum (atmospheric pressure) heat is ap- 1931. Serial No. 542,058.

This succession ofevents is What happens` inV the usual pot operation of nishing caustic soda, requiring an extremely high temperature before a dry iinished product `is produced.

With our process, after 50% NaOH solution originally at room temperature is first placed `under a vacuum of say 28 inches` and heat is applied, the temperature Will rise until, at about 170o (point 2 on the chart),

boiling begins. Boiling continues as heat is applied, With increasing concentration of the liquor to a 60% solution at about 195, to a 70% solution at about 280, and When a temperature of 260o is reached (point 3 on curve A), the temperature ceases torise With further addition of heat, and at this point anhydrous caustic soda crystals form in accordance with further from the solution. In other Words, the crystals form as the Water leaves and the solution remains at approximately 75% concentration until ultimately a dry product is produced, all Without further change of temperature or pressure, which remain respectively at 260 and 28 inches.

Similarly, we have found that if a 50% NaOH solution at room temperature is heated say to2800, and ture is maintained constant a successively increased vacuum is applied, as the vacuum increases boiling begins at about 6 inches of vacuum` (point 4 on the chart), and `if the vacuum is further increased, boiling continues, the solution becomes more concen` trated to 60% and then to 7 0%, until finally evaporation of Water While this temperavalues will 10 anhydrous .caustic soda crystals, and when the respective conditions of temperature and vacuum represented by any point on the line A are reached, further attempts to increase the temperature or the vacuum beyond such have no other eiect than to increase the production of caustic soda crystals, the solution remaining of the strength according to the lines a, b, c and 0l at the particular pointon curve A representing the temperature and vacuum conditions.

The curve A may befregarded as very dependable through that portion indicated by the full line, to wit, Jfrom about 175o to say 400. Between these limits we have demonstrated dependability of the values by many trials in practice, chiefly because the limits of 175o to, say, 400- include the most practical temperature and vacuum values. Be-

yond these limits the curve has been extra-I 39 polated from theoretical considerations and should be regarded as only approximately correct. Of course, the conditionslof temperature and vacuum represented at any point on the curve A can be reached not only by holding the temperature constant and increasing the vacuum or holding the vacuum constant and increasing the tem,r

perature, but also by variations of both. In

other words,

'of this invention to any particular evaporation path, the essential point of the invention being the continuation of evaporation after a point on curve A has'been reached, with the simultaneous crystallization of anhydrous 'caustic soda.

`Between about 145o F. and 400 F. the course of curve A is given fairly accurately by the empirical equation in which'P :inches of mercury vacuum relative to' 30 inches standard, andV t=degrees Fahrenheit. Thisparticular equation has no special significance otherv than that it is an Geiempirieal equation prepared or chosen Jfrom the mathematical standpoint to represent the actual curve A with sufficient accuracy torall practical purposes. 4We prefer to oper-y ate in this range, but because the curve A Gzfouowg approximately the course shown by there is an infinite number of' 19 tem erature and vacuum variation aths the dotted extensions at higher temperatures, evaporation with simultaneous anhydrous caustic crystal formation is possible at any vacuum greater than the highest point of the curve or, in other words, at any vacuum greater than 16 inches, which is the vacuum at approximately 5100.

In the practice of this invention, the solution to be evaporated may be held in any suitablevessel by means of which proper heat and vacuum may be applied to the solution. The process may be carried out, for example, in a closed vessel suitably subjected to heat with vacuum applied to its interior until the entire mass of the solution is converted to anhydrous caustic soda crystals, whereupon the vessel may be opened and the solid crystal mass removed and broken up or powdered for use if necessary or desirable. On the other hand, continuous operation may be desirable, in which case anevaporator may be employed'equippedv for mechanical circulation of the liquid and tor economical application of heat, such as by steam circulating pipes. An evaporator with a circulating propeller in a well is suitable because of the increased .tendency to produce separate and distinct crystals instead of islands of crystals which clog the circulating system. Moreover, with such anev'aporator the operation 4may be carried out continuously with continuous feed of 50% liquor to the evaporator and simultaneously therewith the withdrawal of a more concentrated solution containing anhydrous caustic soda crystals, from which the crystals are separated in any suitablemanner with the mother liquor returned to the evaporator. tends to produce a very pure product because the crystals freeze out from the solution, leaving impurities behind in the mother liquor, which, of course, remains at the concentration represented by the working point on curve A. This mother liquor ultimately collects more and more of the impurities and when the value of the impurities reaches a maximum it must itselt'be specially treated for removal of the impurities or replaced by` case the separating operation is preferably carried out at approximately the same tem perature at which the crystals were formed,

to avoid. the additional production of crystals during the separating operation by any reduction of the temperature. The same reason dictates jacketing et circulating pipes to prevent freezing of the solution therein or the deposit of crystals such as might'require removal by the use of steam or the like. Examples of operations practicing the invention are as follows:

f Example 1.-100 pounds of feed liquor 'of the analysis given below were so evaporated Such a practice be removed by liltration that there remained 62.5 pounds of slurry, at ()o F. and 28.4 inches standard vacuum. On centriiuging` this slurry, there were obtained about l0 pounds of crystals and 52.5 pounds of mother liquor of the analysis shown:

reed @tgl/SS rimane Water 4e. 24 4.15 24. o9 Solids 50. 76 95.85 75.91

Analysis ol solids 99. 34 98. l0 .55 1. 24

same manner exce t that the tem Jerature and pressure were about 260c F. and 27.8 inches vacuum respectively. The crystals and liltrate analyzed:

Ogg' run-ate Water 4. 04 22. 4G Solids 95. 96 77. 54

Analysis ofsolids... 9B. 38 l. 01

The crystals thus obtained are flaky and have a large surface, relative to their mass. Their apparent bulk density is quite low, about 40 pounds per cubic i'oot. Both of these are new and desirable features in commercial caustic sona. The few percent of Water in I the crystal mass does not cause a wet product temperatures being the lower solubility of caustic at low temperatures. As a result, temperatures toward the lower limit of 1450 are more desirable than those toward the upper limit of 400, although the lower temperatures of course require a higher vacuum.

The pressures indicated on the chart refer of course to partial pressures of water vapors. If any other gas or vapor suchas air is present, the total pressure has no significance on the chart. Therefore, assuming evaporation of a 7 0% caustic solution at 300 F. in a vacuum pan, from the chart it will be' seen that the operation will require approximately 20 inches of vacuum. Should this pan be suddenly opened to the atmosphere, the temperay ture and composition remaining constant, the

total pressure, of course, rises to one atmosphere. However, the partial pressure of water vapor remains where it was before, at 20 inches of vacuum. The effect of the extra air pressure on the liquid phase will be practically nil.

`In the practice of the continuous process before mentioned, where the mother liquor from the centrifugal slurry of crystals is returned to the evaporator, it is possible to send the mother liquor or filtrate to a second evaporating pan where a crop of crystals of slightly less purity is obtained, and then from the second pan to a third pan where crystals of still less purity are obtained. The final impure product can then be used for making the so-called Nag() caustic by adding the requisite amount of salt cake. Thus the impurities may be largely concentrated in this lower test product where they are not objectionable.

This method is a material improvement over the ordinary practice of producing anhydrous caustic soda in open pots because it avoids attack of the pots by the strong solu-` tions and high temperatures and high replacement cost for pots. In additiomin the old pot method the corrosion products contaminate the caustic so that it is necessary to treat it with sulfur at high temperatures to precipitate impurities, a practice which is also avoided. Again, the escape of caustic sprays into the atmosphere or other operating conditions dangerous to the workmen are avoided. Again, pots used in ordinary practice finish from four to seven tons of NaOH per day, and larger sizes have proved impracticable, whereas our invention may be carried out in units of ten times this capacity. y Finally, the product is crystalline and grinding is not necessary, while its ability to absorb gases and to dissolve are of ahigh order.

lVhat we claim is:

l. In a process of the type described for separating caustic and water from an aqueous caustic solution, that step which comprises the simultaneous evaporation of water under a vacuum of a value greater than approximately sixteen inches of mercury land crystallization of anhydrous caustic.

2. A process of the type described for separating anhydrous caustic from a water solution, comprising simultaneously evaporating water and crystallizing anhydrous caustic under a vacuum of a value greater `than approximately sixteen inches of mercury, and

separating the caustic crystals from the` mother liquor.

3. Aiprocess of the type described for cril r the caustic is increased, and continuing this process beyond the pointl at which the vacuum ceases to rise, whereby anhydrous caustic crystals are formed.

5. A process of the character described, comprising the vacuum evaporation of caustic soda underv such conditions that the vacuum and temperature are substantially given by the equation:

between the temperatures of 145 and 4000 F., and continuing the evaporation under such conditions, with simultaneous formation of'anhydrous caustic crystals.

6. A process of the character described, comprising the simultaneous evaporation and crystallization of a caustic `soda solution under a vacuum and temperature substantially given by the equation:

between the temperatures of 145o and 4000 F., and separating the resulting crystals from the mother liquor.

7. A process for producing substantially anhydrous caustic soda of increased purity, which comprises evaporating water and simultaneously crystallizing anhydrous caustic from a caustic soda` solution under the conditions of temperature and vacuum given by the equation: Y

between the temperatures of 145"D and 400o F., separating the mother liquor from the crystals while. substantially maintaining their temperature, and cooling the crystals to room temperature.

8. A process of producing anhydrous caustic from solutions thereof, consisting in applying to the solution a vacuum effect at a value greater than approximately sixteen inches of mercury and` av heating eect at a temperature greater than 145o F., increasing at least one ofsaid effects, until stable conditions are reached, and continuing the process for the Aproduction of anhydrous caustic crystals. Y

9. A process of the character described in claim S. in which the vacuum etl'ect is increased to stable conditions.

10. A process of the character described in claim 8 in which the heating effect is increased to stable conditions.

11. A process of the character described in claim 8 in which both of the heating and v acuum effects are increased to stable conditions.

In testimony whereof we hereby affix our signatures.

GEORGE LYNN. RALPH VVADSVVORTIlMILLER. 

